It's much more likely that the students who take the time to watch supplementary materials are the ones who'd do better regardless of the quality of the given material. If you have access to fresh learners, I'd say it's a good opportunity to test how good those series are.
I'm not sure anything I say can convince you, as your mind seems to be made up already, but my experience has been that those videos help my students understand the concepts of linear algebra, comparing the mastery level now vs. before the series was published on YouTube. Especially when it comes to getting a feeling for what a linear transformation is, and how the properties of matrices echo the properties of the transformation they represent.
As for "fresh learners", I can provide my own son, who watched the series before taking his first linear algebra class, and says that it helped him a lot.
I'm not defining education as passing tests. A test is just a measuring instrument.
In fact, your claim is strong and quite easy to test. If watching the series gives people a better fundamental understanding which makes subsequent learning more efficient, take matched novices, randomise them to watch the series or not, then teach both groups some later lin alg materials and measure learning gains on novel problems.
That's the kind of test I mean, not 'can you grind through the course exam'.
If your claimed deeper understanding neither improves what someone can do afterwards nor how efficiently they learn related material, I'm not sure what observable content the claim has.
